Even though the post office can scan your mail, they don't keep a copy of all of it. They can't retroactively scan all of your communications years later. This

Even though the post office can scan your mail, they don't keep a copy of all of it. They can't retroactively scan all of your communications years later. This is an enormous differencs in practice. The same goes for intercepting SMS: unless someone has been targeting you for years, your past messages are safe.